Ants are one
of the few groups of animals which modify their immediate environment
to suit their needs. They build often elaborate nests in a range of
situations, sometimes expending huge amounts of energy in their construction.
These nests are commonly occupied for years and some for decades. In
addition, some ants use plant fibres or soil to construct protective
coverings over nests and feeding areas. Only a handful of animals manufacture
such elaborate and complex structures.

Nests in soil
vary from small, simple chambers under rocks, logs or other objects
on the ground to extensive excavations extending a meter or more into
the soil. The exact structure of the nest varies with the species, soil
type and situation. The entrances to these subterranean nests show a
wide range of styles. Many are no more than a cryptic hole just large
enough for a single worker to squeeze through. Others are a single entrance
surrounded by soil which varies from a low and broad mound to a tall,
narrow turret. A number of species assemble soil and leaves around their
nest entrances to form large piles with well-defined, vertical sides
and concave tops. Others
collect plant material to construct thatched mounds above their subterranean
nests.

The nests of
the common meat ant of south-eastern Australia (Iridomyrmex
purpureus) can grow to enormous sizes with tens of thousands
of workers. They clear all vegetation from the surfaces of their nests
and cover them with small stones. A single colony can be composed of
numerous individual nests separated by up to several hundred metres.
Individual nests can have 10 or more separate, small entrances just
large enough for individual workers to move through.

Some species
of ants nest arboreally. Their nests are most frequently found in twigs,
branches or the trunks of trees. Australian
species are not known to attack firm wood, most utilising the burrows
of other insects such as beetle larvae, or entering rotten wood or wound
sites caused by wind or insect damage. In most cases the entrances to
these types of nests are either small and circular or are formed by
the natural contours of the tree or branch.

In a few arboreal
species nests are constructed using leaves. For example, the green tree
ant (Oecophylla smarigdina)
glues together individual leaves with silk produced by their larvae.
The colony expands both by enlarging existing leaf nests and by adding
new satellite nests. In other arboreal species, plant fibres are used
to construct coverings which are attached to the surfaces of leaves.
The ants live within the chamber formed by the covering and leaves.

While many ants
form elaborate nests, those of other species are relatively simple.
Many of the species found in rotten wood do little more than remove
loose wood fibres to construct simple chambers for workers and brood.
These chambers can be small or very extensive but often lack the complexities
of nests found in soil or arboreally. Finally, a handful of species
(for example some Leptogenys)
lack what would normally be thought of as a nest and
are found in small groups clustered on the ground in leaf litter or
among the roots of plants. These species move their "nests"
frequently and can be found in a wide range of suitable sites.